Recently, standards for third generation (3G) mobile communication systems have being considered in standardization projects, such as, 3GPP: 3rd Generation Partnership Project. One of the topics under consideration is a frame transmission method in downlink (link from a base station to a terminal station).
For example, as shown in FIG. 8, Non-Patent Literature 1 proposes a structure for a downlink layer 1/layer 2 control channel (L1/L2 control channel) in a mobile communication system that divides one cell into a plurality of sectors. In FIG. 8, a base station 100 uses a directional antenna to divide a self cell 110 into three sectors #1, #2, and #3. FIG. 9 is a structure for a downlink sub-frame 200. In FIG. 9, the structure of the downlink sub-frame 200 is expressed using a time axis (Time) and a frequency axis (Freq.). The downlink sub-frame 200 includes an area 201 for storing a reference signal, an area 202 for storing the L1/L2 control channel, and an area 203 for storing user data. The reference signal is scrambled with a scramble code that is specific to the base station (Node B common scrambling code) and stored in area 201. A downlink sub-frame 200 is created for each sector. The base station 100 transmits each downlink sub-frame 200 to its respective sector.
FIG. 10 is the structure of the L1/L2 control channel described in Non-Patent Literature 1. As shown in FIG. 10, L1/L2 control information destined for each of a plurality of terminal stations is subjected to code division multiplexing (CDM). The code-division-multiplexed L1/L2 control channel signal is stored in area 202. In the example of FIG. 10, L1/L2 control channels are transmitted to each of four terminal stations UE_1, 2, 3, and 4, and the L1/L2 control data destined for each of the terminal stations UE_1, 2, 3, and 4 is code-division multiplexed.    Non-Patent Literature: 3GPP, R1-062171, KDDI, NTT DoCoMo, “L1/L2 Control Channel Structure with CDM Based Multiplexing in E-UTRA Downlink”, Aug. 28-Sep. 1, 2006